currently, the are only 40 machines in 19 of the 0 commercial airports in t united states th follow t.s.a. security procedures. 450 more will come onlinthis year to screen passeers for items concealed under layersf clothing. 350 of those were orderewith recently allocated fds. the machines cost upwas of $150,000. t there are concerns from those who claim that t health d privacy of travelers would be at risk. there are two kinds of macnes. the first, mlimeter wave, is a booth that uses harmle electromagnetic was to create a three dimensional image. the other kind, a backatter maine, takes a double x-ray to create a two-sided image dr. hadevappa mahesh, chief physicist at johns hopkins universityospital, showed us how they work. he said the radiion levels, espeally with the backscatter chine, are not high. >> the radiationose levels are ite low. when i say qui low, let's give a comparison witthe respiratory medil chest x-ray, let'say. compared to a medical chesx- ra a typical backscatter, you needo acquire or go through the scanners nearly 000 to 2,000 times fore the dose reaches to a typic